1. Field of the Invention
The present invention relates to an electronically controlled automatic transmission system to be mounted on a vehicle and, more particularly, to an electronically controlled automatic transmission system for applying and releasing their clutches and/or brakes, when the gears for the speed ranges are to be shifted or changed, in accordance with the drive torque of the engine. The present invention further relates to an automatic transmission system of the aforementioned type for bringing the clutches and/or brakes gradually into engagements when the gears are to be shifted.
2. Description of the Prior Art
In a vehicular automatic transmission system disclosed in the prior art, the gears are shifted for the target speed range by once releasing the clutches and/or brakes being applied to engage and then by applying the clutches and/or brakes selectively for the speed range. The engagement and releases of the clutches and/or brakes are accomplished by switching the flows of a working liquid at the solenoid valves by fluid pressure switching means such as oil pressure lines. The solenoid valves are controlled by a microcomputer or the like.
In an electronic timing control, the engine revolutions abruptly rise, if the released clutches and/or brakes are released while the engine drive torque is high. This makes it necessary to adjust the timings for starting the engagements of the clutches and/or brakes. Therefor, the engine drive torque is detected to change the adjustment in accordance with the drive torque detected.
In the system disclosed in the prior art, however, the engine revolutions may abruptly rise if the drive torque detector is troubled or has its output value offset due to aging.
In the vehicular automatic transmission system of the prior art, according to another aspect of the present invention, there is known a technology by which the clutches and/or brakes of the automatic transmission are to be brought not abruptly but gradually into engagements. These gradual engagements are effected by changing the engaging ratio gradually to prevent the abrupt engagements and to reduce the shocks.
In the meanwhile, the clutches and/or brakes at the engagement side may be engaged due to the delay in the oil pressure before the clutches and/or brakes at the release side are completely released. In this dual engagement state, the drive torque will drop, but the completely neutral state is not invited to raise an advantage that few shocks are caused at the engagements of the clutches and/brakes.
However, this dual engagement state cannot be detected very well by the detectors disclosed in the prior art. As a result, the state bypasses the dual engagement to the completely neutral state, or the dual engagements are so high that the drive torque is dropped which causes intense shocks. This makes it impossible to achieve the control using the dual engagements.